This invention relates to television, R.F. circuitry and in particular to R.F. signal generating circuitry at the output stage of a television signal source such as a video disc player, a video cassette recorder or a television game.
Video disc players, T.V. games and other ancillary T.V. signal generating apparatus produce, at their output terminals, signals which are compatible with conventional broadcast television signals so that they may be applied to the antenna terminals of a conventional television receiver. Typically a transfer switch is provided for alternately connecting either the T.V. antenna leads or the output connection of the ancillary T.V. signal source, e.g., TV game, directly to the antenna input terminals of the receiver so that the ancillary signal may be produced on one of the vhf television channels without interference from the broadcast signal assigned to that particular channel. However, if the receiver is located in a geographic area where the broadcast signal allocated to the channel utilized by the ancillary signal source is particularly strong, and the receiver has input circuitry which is highly sensitive, the broadcast signal may interfere with the ancillary signal despite the alternate connection.
In order to overcome such interference these ancillary TV signal generating devices typically have the provision for generating their output signals on two adjacent TV channels. Broadcast TV channels are normally allocated so that for a given geographic area, strong signals will not be present in both of adjacent channels. Thus the user of the ancillary TV signal source will have the option of switching to the one of the adjacent channels least affected by interference.
Broadcast TV signals are transmitted using frequency division multiplexing. Each TV channel is allotted a particular frequency spectrum and the signal bandwidth within the allotted spectrum is prescribed by regulation, e.g., in the United States by Federal Communication Commission regulations. Similarly, other regulations have been promulgated defining the requirements for signals available at the RF output terminals of the ancillary TV signal generating apparatus (see, for example, Code of Federal Regulations, Title 47, Section 15.409). But regardless of differences in regulations between broadcast and ancillary signals, the signals must be compatible for material reproduction on a conventional broadcast receiver.
A base band television signal includes a composite of luminance, chrominance and synchronizing signals with a bandwidth extending from 0-4.25 MHz (NTSC) and an FM sound carrier located at 4.5 MHz. These signals are used to amplitude modulate an RF carrier to produce the broadcast TV signal. The RF signal before transmission is a double sideband signal. Since substantially only one sideband is necessary to reconstruct the signal, part of the lower sideband is eliminated by filtering to produce a vestigal sideband broadcast signal, i.e., in the NTSC system the lower sideband is cut off at 1.25 MHz below the carrier frequency. Elimination of a portion of the lower sideband including the lower sound carrier reduces the necessary per channel bandwidth permitting the allocation of a greater number of channels in a given portion of the frequency spectrum. FCC regulations require that broadcast signals be vestigal sideband, however, the RF output signal from ancillary sources is not limited to quite so narrow channel bandwidths.
Heretofore ancillary TV signal source generating apparatus providing RF signal on two selectable output channels had provision for tuning the frequency of a single carrier oscillator between one of two adjacent frequencies and simultaneously tuning the passband of a series output filter defining the bandwidth of the RF output signal or at least the passband of the sound components of the modulated carrier, see for example U.S. Pat. No. 3,775,555, Carlson. A second technique utilized is to employ two independent oscillator-modulator systems each with a separate vestigal sideband filter tuned to but one channel. (See National Semiconductor 1978 Linear Databook, "LM1889 TV Video Modulator". pp. 10-156.) The latter method is capable of producing a more accurately tuned system but is also the more expensive to assemble. The system has been found difficult to realize in a compact arrangement because attempts to tune one of the channel vestigal sideband filters invariably detunes the other channel filter due to RF coupling of the closely situated circuit elements. The former method incorporating common circuit elements for both channels is less expensive and conceptually more compact (desirable for a volume limited package), but because of the variable tuning, is susceptible to frequency error both in the oscillator and the filter. This arrangement also requires connections between the channel selection switch and the filter which may produce sources of undesirable RF radiation.